EU261 applies to the ticketing carrier when it comes to refunds. The reason for this is that the operating carrier has nothing to refund, per se, as operating carriers generally do not get paid by the ticketing carrier until after the passenger is uplifted. Furthermore, the amount that the operated carrier would be paid would be less than what the passenger paid to the ticketing carrier. When it comes to compensation under EU261, this is applicable to the operating carrier. When you read the regulation, anything relating to compensation/rerouting etc. refers to "operating carrier". Whereas when referring to reimbursement, there is no mention of "operating carrier".

I was in a similar situation last year where I flew FRA LH SIN SQ AKL. The FRA-SIN flight was operated by LH but was codeshared with SQ and was cancelled. LH advised that they are not responsible for the refund but were for compensation. Once raised with SQ, they provided a full refund immediately and even specifically mentioned EU261.